Up till now the modern microelectronics has been developing by way of successively reducing the microcircuit elements from micron to submicron size range. But ever increasing urgent demands in developing nanometric-size elements leads to search for novel techniques of lithographic formation of a conducting structure that assure high resolution which herein implies a minimum size of the elements of the conducting structure under development, that determines a limiting permissible density of a conducting structure elements per unit length or unit area without a contact therebetween.
One prior-art method of forming a pattern by lithographic imaging (cf. U.S. Pat. No. 5,376,505). The method consists in that a substrate carrying a radiation-sensitive material is irradiated, through a mask, with a beam of charged particles, with the result that said material is transformed to form on the substrate a pattern corresponding to the mask pattern stencil. The charged particles are accelerated to a velocity corresponding to de Broglie wavelength and having its maximum value below the preset projection accuracy. The beam is established by particles which move within a certain solid angle along different paths, whereby said particles get incident upon the substrate surface at different angles, too. To attain a preset accuracy of forming a lithographic pattern, resort is made to scanning some mask areas so that the beam changes its position in passing from transparent areas (i.e., mask pattern stencil) to shaded ones, and vice versa.
The beam particles differ widely from one another as to the mean charge value thereof, otherwise speaking, there occurs dispersion as to energy of charged particles. Insofar as such particles are incident on the mask at different angles, the image of the transparent mask areas (i.e., mask pattern stencil) on the substrate becomes blurred. These phenomena prevent high resolution to be attained, that is, make it impossible to obtain separate pattern elements having linear dimensions on the order of unities of nanometers. Moreover, mask scanning provided in the known method sophisticates the process and affects adversely its production output.
Another method of forming a lithographic pattern on a radiation-sensitive surface is known (cf. U.S. Pat. No. 5,561,008) to consists in that a mask with a pattern stencil made therein is once-through exposed to the effect of a beam of charged particles. Once having passed through the mask, said charged beam is focused on a surface (i.e., substrate) carrying a radiation-sensitive material deposited thereon, with the result that said material is transformed to form on the substrate a pattern corresponding to the mask pattern stencil.
It is common knowledge that any source of charged particles is far from being an ideal one because the particles may feature dispersion as to their energy, and the beat itself may have a finite angular divergence. The presence of energy dispersion in charged particles is a source of chromatic aberrations in the image-forming systems and the presence of angular divergence in a charged beam is a source of spherical aberrations. The aforementioned aberration types are, apart from de Broglie wavelength corresponding to the charged particles, a factor which controls a limiting resolution that is attainable in image forming systems.
Thus, high resolution required in microelectronics for fabricating integral circuits and other such products is unattainable unless special restrictions are placed upon the parameters of energy dispersion of a charged beam (chromatic aberrations) and of divergence of a beam incident upon a mask (spherical aberrations).